Paper coating compositions, or coating colors, are used by the paper industry to impart the desired strength and cosmetic properties to finished paper. The coating composition is an aqueous dispersion consisting mainly of mineral pigments like clay, calcium carbonate or titanium dioxide, and pigment binders of natural protein, for example casein or soy protein, starch or synthetic polymer emulsions. Styrene-butadienes and polyvinyl acetates are examples of such synthetic emulsion binders. Coating compositions may also contain low levels of additives, such as thickeners, humectants and lubricants.
Coating compositions are usually applied to a continuous web of material by high speed coating machines, such as blade coaters, air knife coaters, rod coaters and roll coaters. There are trends in the paper industry to use faster coaters to increase productivity and to use higher solids coating compositions to decrease drying costs and improve binder distribution which enhances paper quality.
Polyvinyl alcohol is commonly dissolved in water by heating and added to a high solids aqueous pigment dispersion which is then incorporated into a typical coating color composition. The polyvinyl alcohol portion is commonly used to "carry" fluorescent whitening agents (optical brighteners) in coating color compositions that result in the highest quality printing papers. It is known in the art to add partially or fully hydrolyzed lower molecular weight polyvinyl alcohols as aqueous solutions to such color compositions.
Low molecular weight, fully hydrolyzed polyvinyl alcohol is currently used world-wide as a minor (about 0.5-2 parts/100 parts pigment) but important ingredient in paper coating compositions to carry optical brightners. Typically these compositions are designed for maximum solids. The currently used grades are 98+% hydrolyzed and have a degree of polymerization ranging from 100-600. These fully hydrolyzed, low molecular weight polyvinyl alcohols are "cooked out" in water. i.e., dissolved in water by heating, prior to incorporation into the pigment dispersion. Even though added as a solution at a low level based on total weight of dry ingredients, the additional water incorporated is considered undesirable since the goal is to higher and higher solids coating compositions.